I think "sufficient" doesn't do the 8600k justice. The 8600k is a great choice for high fps setup and the gap to the 8700k is not that big. If running 144Hz or greater you may see small benefits from a 8700k in todays games. Before Coffee Lake released the 7700k was the gaming king and what people recommended for high fps gaming (100Hz is not that high in my view). The 8600k equals or beats the 7700k in the reviews I have read and the gaming landscape hasn't changed. I'd say the £130 is a gamble on future proofing, I am not saying the 8700k is a bad choice, its an awesome choice but I see no hard evidence why its needed, only speculation about possible futureproofing.
In the grand scheme of your build the cost difference is 5%,you may decide to speculate on futureproofing, I just can't say if it pays off.